Laminins are a family of large extracellular matrix (ECM) proteins found primarily in basement membranes associated with all epithelial, endothelial, muscle, fat and Schwann cells. The laminins serve critical functions in cell attachment, growth, migration, and differentiation of many cell types. Laminin I is the first extracellular matrix protein to appear during embryonic development, where it surrounds the inner cell mass of the compacted blastocyst. Studies of laminin I purified from the Engelbreth-Holm-Swarm (EHS) tumor established that laminin is required for cell attachment and growth, and many studies confirm the importance of laminins in development and survival. Laminin interacts with cells through a variety of integrins, the dystroglycan receptor, syndecan, and other type receptors broadly expressed on many cell types.
Extracellular matrix (ECM) provides the extracellular environment for almost all mammalian cell types. It is composed of structural proteins such as collagen and elastin, proteoglycans, and proteins such as fibrin, fibronectin, and laminin. One of the over-reaching goals of cell biology and tissue engineering is to recreate the extracellular environment a cell experiences in vivo, and attaining the appropriate ECM components in appropriate morphological and physical characteristics is of the utmost importance.
Peripheral nerve transection occurs commonly in traumatic injury, causing motor and sensory deficits distal to the site of injury. Transection requires appropriate surgical intervention to maximize retention of function and sensation [1]. Reanastomosis by direct suture of the severed nerve fiber endings through the perineurium is the gold standard and results in the best surgical outcome; however, when the nerve retracts after injury and tensionless repair is impossible, cable grafts are often used. Cable grafting takes short nerve segments from a donor nerve and directly reapposes a series of grafts to fill the nerve gap without tension [2]. This procedure leaves deficits at the donor site, and is variably less successful at recovering function at the injury site. To help alleviate donor site morbidity, increased operative time, and size mismatch of the donor nerve, clinicians may choose a nerve conduit for repair of sensory nerves. Conduits currently on the market are biocompatible, biodegradable, hollow tubes into which the nerve ends are sutured. These conduits serve as only an empty, isolated space for growth. Regeneration through nerve conduits typically provides an improvement over no treatment, but for long defects (≧10 mm), conduits often fail due to lack of structural support over the time required for the axon to traverse the gap distance [3].
When axons remain without connection to their target tissue over significant periods of time they lose the ability to regenerate, and the possibility for functional recovery is lost. A decline in the regenerative capacity of both axons and Schwann cells, the support cells of the PNS, begins in humans approximately eight weeks after injury. At six months to one year, regeneration is much less likely [4]. This knowledge of the degeneration and regeneration processes has led researchers to the conclusion that, to outperform autografts and allografts, conduits must provide structural support to regenerating axons [5]. To facilitate increased speed of regeneration, in addition to physical support and guidance, the ideal conduit would also provide biochemically relevant signals to guide axonal outgrowth, thus playing an active role in peripheral nerve regeneration.
Multiple strategies exist for improving repair and regeneration with nerve conduits. These involve optimization of cellular components, extracellular matrix proteins, and soluble factors. As occurs in vivo, the presence of any one of these three can cause generation of the other two. Extracellular matrix proteins not only present appropriate and recognizable surfaces for interactions such as cell binding and migration, but are able to be manipulated and remodeled by cells to match a more uninjured milieu. Utilizing extracellular matrix components allows for natural cell-matrix interactions to occur such as ligand binding, process guidance, and regeneration, as the substrate can drive cell-fate decisions [6]. These cell-fate decisions in vivo are driven by interactions with the dynamic tissue matrix within the extracellular environment.
Electrospun laminin nanofibers can function as a basement membrane mimetic material, both in terms of geometry and composition, driving attachment, differentiation, and process extension of neuron-like or neuronal precursor cells [7]. Electrospinning is an ideal technology to create implantable 3-D scaffold conduits for peripheral nerve regeneration. The resulting isotropic randomly oriented nanofibrous mesh, or anisotropic aligned nanofibrous mesh will provide the necessary structural support and high surface area to volume ratios to facilitate cell migrations required to bridge peripheral nerve to aid in regeneration. Other groups, notably Bellamkonda and colleagues [3,8,9] have filled conduits with thin films of synthetic polymer fibers and found this physical support for outgrowth, along with directional guidance through fiber alignment, support regeneration and functional recovery across long gaps (>10 mm).
There is a long felt need in the art to recreate an extracellular environment to aid in cellular and tissue processes such as attachment, migration, and wound healing. More specifically, there is a need to create such an environment to enhance nerve regeneration. The present invention satisfies these needs.